As summer vacation begins, Ryuuki, a kind-hearted student devoted to his older sister Reiko, finds himself unexpectedly captivated by a famous erotic streamer named Kirill. A chance encounter leads to a provocative meeting where Kirill decides to guide Ryuuki through the loss of his virginity. After an unforgettable and filmed experience, Ryuuki is left reeling from his sudden awakening as he returns to his ordinary life.
Ryukki leads his friends into the woods to reveal a secret video of his encounter with the provocative Kiriru, only for her to appear in person. As punishment for betraying their secret, Kiriru forces Ryukki to film her engaging in an orgy with his friends. Amidst the chaos and jealousy, Ryukki's passion for Kiriru grows, eventually leading the two to start a romantic relationship.
After a triumphant soccer match, Ryukki finds himself caught between his manipulative girlfriend, Kiriru, and his childhood friend and team manager, Chiaki. Under Kiriru's provocative influence, Ryukki and Chiaki engage in a series of daring and explicit encounters across the school grounds, leading Chiaki to surrender her innocence in the most risky of locations.
Ryuuki discovers the shocking truth that his sister, Reiko, is the secret identity behind the persona "Kirill." After Reiko confesses her struggle between her responsible sisterly role and her repressed desires, Ryuuki accepts her completely, leading the pair to embrace their taboo relationship through daring public encounters. As the summer draws to a close and Reiko returns to Tokyo, Ryuuki reflects on his transition into adulthood and the secret bond they now share.